Marginal oil wells are an often overlooked, but vitally important segment of the North American petroleum industry. Secondary recovery or enhanced oil recovery (EOR) encompasses a variety of techniques designed to increase oil extraction/recovery from an existing well. In a typical oil well, pressure in an underground formation pushes oil upward, allowing it to be extracted. In older wells and mature fields, this pressure diminishes over time, decreasing the flow of oil. Typically, due to the significant drop in pressure, 30-70% of the original oil reserves remain untapped, as they are more difficult to recover using traditional (i.e. non-EOR) recovery methods.
Marginal or depleted wells have low oil output, but they still provide 17% of oil produced onshore in the US. Without these wells the US would have to increase imports by nearly 7% to make up for the shortage. While each individual well contributes only a small amount of oil (on average 1-3 barrels a day), there are roughly 400,000 of these wells in the United States and less than that in Canada. Combined, these marginal wells produced more than 321 million barrels of oil in 2005, according to the 2006 Full Marginal Well Report of the Interstate Oil and Gas Compact Commission.
Chemical EOR methods include polymer, surfactant/polymer (variations are called micellar-polymer, micro-emulsion, or low tension), water-flooding, and alkaline (or caustic) flooding. All these methods involve mixing chemicals in water prior to injection. Therefore, these methods require conditions that are very favorable for water injection: low-to-moderate oil viscosities, and moderate-to-high permeabilities. Hence, chemical flooding is used for oils that are more viscous than those oils recovered by gas injection methods but less viscous than oils that can be economically recovered by thermal methods. Reservoir permeabilities for chemical flooding need to be higher than for the gas injection methods, but not as high as for thermal methods. Since lower mobility fluids are usually injected in chemical floods, adequate injectivity is required. In most cases, reservoir brines of moderate salinity with low amounts of divalent ions are preferred since high concentrations may interact unfavorably with the injected chemicals.
Other methods include flue gas flooding, which is expensive, requiring extensive equipment to inject high pressure CO2 gas or some other combination (e.g. steam/CO2) into the well.
In some instances, only one type of enhanced recovery technique is applicable for a specific field condition but in many instances, more than one technique is possible. The selection of the most appropriate process is facilitated by matching reservoir and fluid properties to the requirements necessary for the individual enhanced oil recovery techniques. A summary of the technical screening guides for the more common EOR processes compared to developed formulation is given in Table 1.
TABLE 1Summary of Screening Criteria for EOR MethodsOilOilFormationDepthTemperatureMethodViscosityCompositionSaturationTypeft° F.CO2<10 cpC2-C12>30%Sandstone or>2,000Not criticalcarbonateEOR<30Light>30%Sandstone<8,000<200FormulationIntermediatesAnd carbonateAlkaline<200Some organicWaterfloodSandstone<9,000<200acidsresidualpreferredPolymer<150Not critical>10%Sandstone<9,000<200preferredCombustion<1,000Asphaltic>40-50%Sand high>500>150porositypreferredSteamflooding>20Not criticalVariableStone with300-5,000Not criticalhigh porosity
A distinction is made between the oil properties and reservoir characteristics that are required for each process. For example, steam flooding is applicable for very viscous oils in relatively shallow formations. On the other extreme, CO2 and hydrocarbon flooding work best with very light oils at depths that are great enough for miscibility to be achieved. Chemical flooding processes are applicable in low to medium viscosity oils; depth is not a major consideration except, at great depths, the higher temperature may help the extraction and dissolution processes.
Generally, shallow reservoirs are easy to work on they are relatively uniform with reasonable oil saturations, minimum shale stringers, and good areal extent.
Implementation of EOR process is expensive, time-consuming, and people-intensive. Substantial costs are often involved in the assessment of reservoir quality, the amount of oil that is potentially recoverable, laboratory work associated with the EOR process, computer simulations to predict recovery, and the performance of the process in field situation.
One of the first steps in deciding to consider EOR is, of course, to select reservoirs with sufficient recoverable oil and areal extent to make the venture profitable.
With any of the processes, the nature of the reservoir will play a dominant role in the success or failure of the process. Many past failures of commercial EOR products in the market have resulted because of unknown or unexpected reservoir problems. Thus, a thorough geological study is usually warranted.
There are documented limitations for various methods. For example CO2 flooding results in poor mobility control due to very low viscosity of CO2 gas and also the cost and availability of CO2.
It has been reported that breakthrough of CO2 causes corrosion in the producing wells; re-pressuring of CO2 for recycling; and a high requirement of CO2 per incremental barrel produced.
Polymer flooding requires high polymer concentration to achieve the desired mobility control. It is easily adsorbed and lost in clay formation. Also, it does not work well with high water levels and degrades in salinity and presence of divalent ions.
Alkaline flooding has many limitations, and is affected by carbonated formation. It produces scaling and plugging in the producing wells, requires high caustic consumption, and is thus environmentally friendly.
The combustion approach suffers from adverse mobility ratio, is a complex process, requires large capital investment, and is difficult to control. Produced flue gases present environmental problems.
The mechanism of steam flooding is basically heating the crude oil and reducing its viscosity and supplying sufficient pressure to drive oil to the producing well. There are several limitations to the method. Oil saturations must be quite high and the pay zone should be more than 20 feet thick to minimize heat losses to adjacent formations.
Steam flooding is applicable to viscous oils in massive, high permeability sandstones or unconsolidated sands. The method works best with shallow wells to reduce heat loss and costs of the operation. It is also recommended not to be used in carbonate reservoirs. Note that about one-third of the additional oil recovered is consumed to generate the required steam, so the cost per incremental barrel of oil is high.
There are a number of patent and non-patent references that describe the current state of the art of EOR. These include:    U.S. patent application 20090205823    U.S. Pat. No. 7,262,153    U.S. Pat. No. 7,229,950    U.S. Pat. No. 7,137,447    U.S. Pat. No. 6,302,209    U.S. Pat. No. 6,818,599    U.S. Pat. No. 6,022,834    Wubs, H. J., Beenackers, A. A. C. M (1994), Kinetics of H2S absorption into aqueous ferric solutions of EDTA and HEDT, American Institute of Chemical Engineers Journal, 40, 433-444.    Demmink, J. F.; and Beenackers, A. A. C. M (1998). Gas desulphurization with ferric chelates of EDTA and HEDTA; Industrial and Engineering Chemistry Research, 37, 1444-1453.    L. M. Frare et al, Environmental Progress and Sustainable Energy, Vol. 29, No. 1, April 2010    Alberta Chamber of Resources, Oil sands technology roadmap; unlocking the potential, Edmonton, Canada, 2004    Alberta Energy Utilities Board (AEUB), Alberta's reserves 2003 and Supply-Demand outlook 2003-2013, Calgary, 2003.